Motors are used to provide motive forces for many applications. In some applications, a single device is tasked with both rapid, low precision movement during a first activity, and slow, precisely controlled movement during a second activity. In order to achieve both types of movement, many devices must use two motors, one for the fast movement and another for the slow movement, thus increasing the cost, size and weight of the device. Other options include the use of stepper motors, which are able to develop significant slow motor speed torque and precise rotational control. However, stepper motors have numerous disadvantages, including high cost, the need for relatively complicated control circuitry, large size, large weight and high power requirements.
The disadvantages of presently available motor systems are particularly problematic in medical infusion pump systems. Intravenous infusion therapy is prescribed where it is desirable to administer medications and other fluids directly into the circulatory system of a patient. Medical infusion pumps have typically employed stepper motors to provide rotational motive force, and thus are limited by the disadvantages of a stepper motor as listed above. Further, for many clinical procedures it is desirable to administer several medical fluids to a patient simultaneously, thus requiring multiple independent gravity flow controllers and/or multiple independent electronic pumps. The use of multiple independent controllers or pumps, however, is disadvantageous for many reasons, including: the increased possibility of infection occasioned by multiple IV venipuncture; the increased discomfort to the patient, the considerable labor and time required for administering multiple IVs and setting up multiple controllers/pumps; the increased clutter around the patient; the comparatively high cost of procuring and maintaining several pumps; and the comparatively high cost incurred in maintaining an inventory of tubes required by each of the different pump types.
Past attempts to overcome some of these above-described difficulties have resulted in devices utilizing multiple valves, such as described in U.S. Pat. No. 4,696,671 to Epstein et al. Epstein et al. disclose a sterile, disposable cassette containing fluid input and output lines and chambers, wherein the cassette is inserted into a pumping mechanism such that plungers from the pump mechanism engage and close the valves of the cassette. Thus, Epstein et al. disclose a system where the valves of the cassette are biased to an open state when not engaged by a pump mechanism. This arrangement may result in unintentional fluid flow to a patient when the valve set is mistakenly removed from or incorrectly aligned within the pump unit. The valves of Epstein et al. must also be opened sequentially in order to reach the valve of interest. Therefore, an additional valve and independent motive force for the valve is required to prohibit flow during a period in which the unintended valve is open, such as by using a third motor. In some circumstances, operation of valves in accordance with the teachings of Epstein et al. is not desirable due to the resultant fluid interactions or fluid damage, such as cell damage to blood cells.